1. Field of the Invention:
The present invention relates to a channel selecting apparatus in a receiver. More specifically, the present invention relates to a channel selecting apparatus in a transceiver employing a frequency synthesizer.
2. Description of the Prior Art:
As well known, in a receiver such as a transceiver, it is required to generate a local oscillation signal in order to provide an intermediate frequency signal in a reception mode of the receiver. It is further required that such local oscillation signal be changed in frequency depending on the frequency of a channel whose reception is desired. To that end, a frequency synthesizer including a phase locked loop has been advantageously employed in transceivers, inasmuch as it can provide a much more stabilized oscillation frequency.
A frequency synthesizer employing a phase locked loop usually comprises a voltage controlled oscillator the oscillation frequency of which is controllable as a function of an output voltage, as low pass filtered, obtained from a phase detector, which is adapted to compare the phase or the frequency of the output from a reference oscillator and the phase or the frequency of an output from a programmable frequency divider adapted to frequency divide the output frequency from the said voltage controlled oscillator at the frequency division rate which is adapted to be variable as a function of a control signal. Automatic scanning of the oscillation frequency of the output from the said voltage controlled oscillator is effected by varying the said control signal and thus the frequency division rate of the programmable frequency divider.
A conventional transceiver is structured to include a push-button switch for the purpose of selecting a channel, for communication. More specifically, a typical conventional transceiver comprises an upward directional scanning pushbutton switch for causing variation of the tuning frequency, or the channel, in the upward direction and a downward directional scanning push-button switch for causing variation of the tuning frequency, or channel, in the downward direction. One type of a conventional transceiver is structured such that scanning is effected on a one by one basis in terms of channels in the upward or downward direction responsive to depression of an upward or downward push-button switch for a relatively short time period. However, this requires that the push-button switch be depressed repeatedly many times to select a desired channel which is far by many channels from the channel which has been presently selected, with the result of tiresomeness of the channel selecting operation.
In order to eliminate tiresomeness of channel selecting operation on a one by one basis in terms of channels in the scanning operation, a so-called fast scanning of the channels may be advantageously adopted. A transceiver of a fast scanning type is structured such that depression of an upward or downward scanning push-button switch for more than a predetermined time period makes an automatic consecutive channel selection in a fast scanning rate. Nevertheless, a conventional fast scanning scheme has been merely to achieve a rapid rate of channel scanning.
An automatic search scheme has also been proposed wherein the above described fast scanning of the channels is automatically stopped responsive to a signal received in a given channel. However, an automatic search scheme cannot be achieved in a simple fast scanning scheme. More specifically, in order to achieve automatic search, it is required to provide separate mode switches for selectively setting fast scanning mode and an automatic search mode. Alternatively, it is required to provdide a particular push-button switch for setting an automatic search mode. Thus, in order to achieve an automatic search scheme, it is required to provide separately a particular switch for setting an automatic search mode. However, this makes the structure complicated and the cost expensive.
By way of another example, a system may be considered wherein each depression of one switch causes advancement of one channel while each depression of the other switch causes advancement of a predetermined plurality of channels, say ten channels, whereby the channels can be advanced selectively in a fine manner or a coarse manner. As a result, in order to reach a relatively remote channel from the channel which has been presently selected, the said other switch is depressed for the purpose of coarse advancement of the channels and thereafter the said one switch is depressed for the purpose of fine advancement of the channels. Nevertheless, such channel selecting operation is still tiresome and is not particularly preferred in receivers which are very often employed in vehicles such as automobiles.